The present studies further characterize natural killer (NK) cells in the rat. The results demonstrate substantial differences in the ability to recirculate between the cells which mediate NK activity or large granular lymphocytes (LGL) and normal rat T cells. Studies on lymphokine-activated killer (LAK) cells indicate that the major precursor of LAK activity in the rat is an LGL. Studies with normal rat LGL and the LGL tumor lines have demonstrated a lack of T cell receptor and interleukin 2 (IL2)- receptor gene expression, even in LGL tumor lines which respond to IL2 stimulation. Analysis of the granules from LGL have led to the isolation of a cytolytic protein (cytolysin), at least two BLT- esterases, LGL specific chondroitin sulfate A glycosaminoglycans and proteoglycans, and an anti-fungal molecule which inhibits the growth of Cryptococci neoformans. These differences between the LGL and T cells in a number of basic biological properties suggest that LGL represent an entirely separate, third lineage of lymphocytes, distinct from both T and B cells.